The union representing the city’s department-store workers warned of “massive” layoffs after the first of the year.

“I can tell you absolutely we are anticipating job loss. Right now we’re facing the worst shopping season in memory,” said Jim Grossfeld of the 100,000-member Retail, Wholesale and Department Store Union.

As for the scale of layoffs, he said, “It very well could be massive.”

Mayor Bloomberg said he learned of the impending job cuts this week.

“The unions have been notified that right after the first of the year there will be massive layoffs because there’s just nobody going into the stores,” he said on his WOR radio show.

That will have an impact on the city’s tax base, Bloomberg warned.

“And people say I don’t believe it until I see it,” he added. “You don’t have to wait. Just take a look. There’s no economic activity.”

Retail workers said despite retailers’ aggressive discounting, there are signs of the grim holiday season all around, including a decline in foot traffic in stores.

Also, stores normally bulk up their staffs during December. But there’s been “very little hiring of seasonal help,” Grossfeld said. “Stores are not restocking their shelves as they would.”

Bloomberg said he was struck by retailers’ price-cutting.

He said he was at a Modell’s last Saturday to buy presents for needy kids.

“Everything in the store [was] 50 percent to 75 percent off – and this is three weeks before Christmas,” he said.

Mitchell Modell, president and CEO of Modell’s Sporting Goods, said, “We have January prices in December. We’ve never done that in 120 years.”

But Modell said foot traffic in his stores has been good and the bleak prognosis for the city’s retailers could change in the final week of the shopping season.